[shadcn] Project Base Separator into the shadcn design language

Classification

• Ledger: shadcn design-language projection
• Base owner: P-BASE-SEPARATOR (status: draft)
• New Base protocol: none

Goal

Project the current merged Base Separator protocol into @proto.ui/prototypes-shadcn while preserving its contentless, passive semantic/decorative boundary.

Contract boundary

• Base owns horizontal/vertical orientation and decorative versus semantic accessibility projection.
• Decorative mode remains hidden, roleless, non-focusable, and omits orientation.
• Semantic mode projects the separator role and live orientation.
• The shadcn projection owns border/color/spacing presentation only.

Delivery surface

• packages/prototypes/shadcn/src/separator/
• P-SHADCN-SEPARATOR
• exact package/root exports and public CLI registry projection
• packages/prototypes/shadcn/test/separator.test.ts
• EN/ZH docs at apps/www/src/content/docs/{en,zh-cn}/ui-libraries/shadcn/separator.mdx
• real demo apps/www/src/content/docs/zh-cn/demo-shadcn-separator.demo.ts
• preview registry, component preset/token closure, and generated Agent projection

Acceptance criteria

• The projection consumes Base Separator and returns a contentless renderer.
• Decorative/semantic and horizontal/vertical live transitions remain executable.
• No focus, tab stop, event, command, action, mutable value, or authored child surface is introduced.
• Exact exports and default presentation are asserted.
• Docs distinguish Base accessibility guarantees from shadcn styling.
• The real package export renders through Web Component, React, and Vue previews.
• Focused tests, catalog, preset/token, types, docs, and proportional workspace checks pass.

Out of scope

• Resizing, splitting, dragging, or menu-separator behavior.
• Changing empty-string accessibility-attribute omission semantics.